(1) Field of the Invention
The present invention relates to coin telephones and more particularly to a circuit which overrides the initial rate circuit to allow operation of the coin relay independent of whether or not the initial rate has been deposited.
(2) Description of the Prior Art
Many coin telephones in current usage are designed to operate in prepay, and emergency modes of service. In the prepay mode, coins equalling at least a minimum initial rate are required to be deposited before the calling device is enabled. After the required initial rate has been deposited, the calling party can dial the called party or an operator, if this initial deposit is sufficient to place a call to the called party's location, the two parties will be connected. If the initial deposit is not sufficient, the call will be routed to an operator who will connect the two parties upon deposit of the required additional amount. Upon completion of the call or upon detection of a busy condition, the central office applies coin battery, .times.130 V to cause coin collection or coin refund respectively. However, in the event that the caller abandons his call after depositing coins totaling less than the required initial rate, the coin relay must be connected to the central office so it can be operated to refund the coins that have been deposited. Since the rate relay must be operated to connect the coin relay to the central office, the initial rate override circuit responds to the application of coin battery by the central office to override the initial rate circuit and operate the rate relay. The coin relay is then connected to the central office and operates in response to the coin battery to refund the deposited coins.
In the emergency coin free calling service, with the exception of predetermined telephone numbers, the initial rate must be deposited in order for the call to be completed. For predetermined numbers and calls to operators no coin deposit is required. The initial rate override circuit is needed for this mode of service to refund coins in the event that coins deposited are less than the initial rate required.
This function has traditionally been performed by including a neon lamp in a circuit between the central office and the rate relay, such that coin battery would cause the neon lamp to conduct current to the rate relay which would then be operated allowing coin battery to be applied to the coin relay. This arrangement is shown in U.S. Pat. No. 3,760,101 which issued on Sept. 18, 1973 to R.V. Burns. A more modern approach has been to use a triac which fires in response to coin battery to operate the rate relay. This approach is disclosed in U.S. Pat. No. 3,798,470 which issued on March 19, 1974 to R.V. Burns, et al. However, both of these approaches consume substantial amounts of power, are susceptible to oscillation and operation due to transients and cannot withstand high voltage transients.
Accordingly, it is the object of this invention to provide an economical, low cost, highly reliable initial rate override circuit which utilizes low-power integrated circuitry and is not susceptible to oscillation or high voltage electrical transients.